This invention relates to propellers, and in particular to such propellers which have an even number of propeller blades greater than four disposed in opposite pairs, i.e., with the two blades of each pair being arranged in longitudinal alignment with each other at opposite sides and radially of the axis of rotation of the propeller.
It has long been known that propellers are efficient aircraft propulsors in the low to medium flight speed range, i.e., at cruising speeds below about 300 km/h. In this connection, it has been determined that the noise generated by a propeller-driven aircraft is predominantly propeller noise the level of which depends primarily on the Mach number of the blade tips of the propeller. However, the noise generated by a propeller rotating at a speed corresponding to a blade-tip Mach number in the range of about 0.4 to 0.9 becomes a source of annoyance to people on the ground under the flight path of the aircraft and especially to people living in houses in the vicinity of an airport where the frequency of overflights at times is very high. This has led, of late, to the imposition, by both national and local government agencies, of relatively stringent propeller-aircraft noise certification limits (ICAO ANNEX 16/Chapter 10), which has in turn led to the development and use of "low noise propellers."
Prior to the invention disclosed in the aforesaid application Ser. No. 259,854, a significant propeller noise reduction was attainable only by significantly lowering the helical blade-tip Mach number, which required either reducing the propeller speed of rotation or reducing the lengths of the propeller blades. However, the utilization of these expedients to achieve lower blade-tip Mach numbers entailed rather severe performance penalties, primarily in the form of a loss of propeller thrust. Although steps can be taken to compensate for such loss of thrust, for example, by lengthening the propeller blades as an adjunct to a reduction of their speed of rotation, or by modifying the profiles of the propeller blades or providing them with lift aids as an adjunct to a reduction of the blade length, the requisite aircraft modifications to accommodate the alterations of the propeller and its drive mechanism are quite complicated and very expensive to put into effect.
The basic principle of the invention disclosed in the said prior application Ser. No. 259,854 was that a certain degree of propeller noise reduction can be achieved with the aid of a propeller having an unsymmetrically distributed even number (at least four) of propeller blades which are arranged around the common axis of rotation in opposite pairs, i.e., pairs having the two blades thereof longitudinally aligned with each other (180.degree. apart) and disposed on diametrically opposite sides of the axis of rotation. In the case of a four-bladed propeller, therefore, in which a symmetrical blade spacing would have 90.degree. angles between the circumferentially adjacent blades, the two pairs of blades were described as being offset in relation to one another so as to define therebetween a spacing angle .epsilon. of between about 15.degree. and 50.degree., with the magnitude of the spacing angle selected for any given propeller speed that corresponds to a bladetip Mach number within the 0.4 to 0.9 range being inversely related to the magnitude of the blade-tip Mach number. The effect of this arrangement was a reduction in the level of the rotational sound harmonics by interference, with the noise reduction being on the order of magnitude of up to 4 dB(A).
Physical propeller blade arrangements characterized by plural pairs of longitudinally aligned pairs of blades are per se well known in the art; representative arrangements are shown, for example, in U.S. Pat. Nos. 921,423, 1,073,413, 1,944,525, 2,126,221, 3,023,813, 3,592,559, 3,830,587, 4,483,658 and 4,676,459. The prior art never suggested, however, the provision, for purposes of noise reduction, of a propeller with such paired propeller blades having the pairs of blades offset from one another by a spacing angle the magnitude of which is both within the range of about 15.degree. to 50.degree. and inversely related in a precalculated manner to the magnitude of the propeller blade-tip Mach number within the range of 0.4 to 0.9.